Simulating the Dynamics of Magmatic Hydrothermal Systems in Restless Volcanoes: Insights into the Effect of Faulting at Campi Flegrei Caldera

Abstract:

Magmatic hydrothermal systems are the superficial manifestation of high heat flux in groundwater systems and their dynamics can be intermittently perturbed by those of the magmatic system. The complex interplay between heat and fluid flow in hydrothermal systems must be understood in order to discriminate geophysical signals of magmatic unrest from purely hydrothermal ones. Starting from the Campi Flegrei model of Todesco et al. (2010), we investigate the impact of major geological discontinuities in permeability on: advective flow within the hydrothermal reservoir, subsurface pressure and temperature distributions and the relative contribution of volcanic and surface derived fluids to surficial discharge. In the baseline scenario (no faults), a steady state convective flow system develops within 4 ky in which the injection of hot fluids feeds a narrow plume (fumarole), which entrains water from the surrounding aquifer and depresses isotherms by up to 500 m in the zone 400-1500 m from fumarole.

The addition of two steep faults 4 and 7 km from the fumarole (faults A and B) two orders of magnitude higher vertical permeability than the matrix, divides the flow field into three separate advective cells (fumarole, fault A and fault B). Faults focus recharge of shallow groundwater to the deeper reservoir, with the concomitant upwelling of hot water around faults leading to local thermal anomalies (+50 ⁰C) at 500 m depth. Increasing fault permeability by an order of magnitude enhances this effect around fault B, but reverses the flow pattern at fault A which now serves as a conduit discharging fluids at up to 90 ⁰C. Increasing matrix permeability results in interaction between the previously separate advective cells. The juxtaposition of low permeability rock at the faulted caldera margin focusses discharge of hot waters, fed by recharge via faults within the caldera. Simulations also evaluate the effect of unrest, highlighting the impact of fluid flow on subsurface pressure and temperature distribution. For instance, a rapid increase of basal heat flux of two orders of magnitude, to simulate replenishment of a deep magmatic reservoir, induces liquid-gas phase transition at depth, which leads to a maximum pressure of 1.25 MPa at 150 m depth in 150 years. Overpressure is then quickly released by discharge of fluids at the surface.